Abnormal patterns of electrical conduction in the heart can produce abnormalities in the cardiac cycle known as arrythmias. A common form of arrhythmia, known as atrial fibrillation, is a pervasive problem in modem society. Atrial fibrillation is associated with an increased risk of myocardial ischemia, especially during strenuous activity, and has also been linked to congestive heart failure, stroke, and other thromboembolic events.
In the human heart, normal cardiac rhythm is maintained by a cluster of pacemaker cells, known as the sinoatrial (“SA”) node, located within the wall of the right atrium. The SA node undergoes repetitive cycles of membrane depolarization and repolarization, thereby generating a continuous stream of electrical impulses, called “action potentials.” These action potentials orchestrate the regular contraction and relaxation of the cardiac muscle cells throughout the heart. Action potentials spread rapidly from cell to cell through both the right and left atria via gap junctions between the cardiac muscle cells. Atrial arrhythmias result when electrical impulses originating from sites other than the SA node are conducted through the atrial cardiac tissue.
In some cases, atrial fibrillation results from perpetually wandering reentrant wavelets, which exhibit no consistent localized region(s) of aberrant conduction. In other cases, atrial fibrillation may be focal in nature, resulting from rapid and repetitive changes in membrane potential originating from isolated centers, or foci, within the atrial cardiac muscle tissue. These foci exhibit consistent centrifugal patterns of electrical activation, and may act as either a trigger of atrial fibrillatory paroxysmal or may even sustain the fibrillation. Recent studies have suggested that focal arrhythmias often originate from a tissue region along the pulmonary veins of the left atrium, and even more particularly in the superior pulmonary veins.
Several surgical approaches have been developed for the treatment of atrial fibrillation. For example, Cox, J L et al. disclose the “maze” procedure, in “The Surgical Treatment Of Atrial Fibrillation. I. Summary”, Thoracic and Cardiovascular Surgery 101(3):402–405 (1991) and “The Surgical Treatment Of Atrial Fibrillation. IV. Surgical Technique”, Thoracic and Cardiovascular Surgery 101(4):584–592 (1991). In general, the maze procedure is designed to relieve atrial arrhythmia by restoring effective SA node control through a prescribed pattern of incisions about the cardiac tissue wall. Although early clinical studies on the maze procedure included surgical incisions in both the right and left atrial chambers, more recent reports suggest that the maze procedure may be effective when performed only in the left atrium (see for example Sueda et al., “Simple Left Atrial Procedure For Chronic Atrial Fibrillation Associated With Mitral Valve Disease” (1996)).
The left atrial maze procedure involves forming vertical incisions from the two superior pulmonary veins and terminating in the region of the mitral valve annulus, traversing the inferior pulmonary veins en route. An additional horizontal incision connects the superior ends of the two vertical incisions. Thus, the atrial wall region bordered by the pulmonary vein ostia is isolated from the other atrial tissue. In this process, the mechanical sectioning of atrial tissue eliminates the atrial arrhythmia by blocking conduction of the aberrant action potentials.
The moderate success observed with the maze procedure and other surgical segmentation procedures have validated the principle that mechanically isolating cardiac tissue may successfully prevent atrial arrhythmias, particularly atrial fibrillation, resulting from either perpetually wandering reentrant wavelets or focal regions of aberrant conduction. Unfortunately, the highly invasive nature of such procedures may be prohibitive in many cases. Consequently, less invasive catheter-based approaches to treat atrial fibrillation have been developed.
These less invasive catheter-based therapies generally involve advancing a catheter into a cardiac chamber, such as in a percutaneous translumenal procedure, wherein an energy sink on the catheter's distal end portion is positioned at or adjacent to the aberrant conductive tissue. Upon application of energy, the targeted tissue is ablated and rendered non-conductive.
These catheter-based methods can be subdivided into two related categories, based on the etiology of the atrial arrhythmia. The first category includes various localized ablation methods used to treat focal arrhythmias by targeting the foci of aberrant electrical activity. Accordingly, devices and techniques have been disclosed which use end-electrode catheter designs for ablating focal arrhythmias centered in the pulmonary veins, using a point source of energy to ablate the locus of abnormal electrical activity. Such procedures typically employ incremental application of electrical energy to the tissue to form focal lesions. The second category includes methods designed for the treatment atrial fibrillations caused by perpetually wandering reentrant wavelets. Such arrhythmias are generally not amenable to localized ablation techniques because the excitation waves may circumnavigate a focal lesion. Thus, the second category of catheter-based approaches have generally attempted to mimic the earlier surgical segmentation techniques, such as the maze procedure, wherein continuous linear lesions are required to completely segment the atrial tissue so as to block conduction of the reentrant wave fronts.
An example of an ablation method targeting focal arrhythmias originating from a pulmonary vein is disclosed by Haissaguerre et al. in “Right And Left Atrial Radiofrequency Catheter Therapy Of Paroxysmal Atrial Fibrillation” in J. Cardiovasc. Electrophys. 7(12):1132–1144 (1996). Haissaguerre et al. describe radiofrequency catheter ablation of drug-refractory paroxysmal atrial fibrillation using linear atrial lesions complemented by focal ablation targeted at arrhythmogenic foci in a screened patient population. The site of the arrhythmogenic foci was generally located just inside the superior pulmonary vein, and was ablated using a standard 4 mm tip single ablation electrode.
Another ablation method directed at paroxysmal arrhythmias arising from a focal source is disclosed by Jais et al. “A Focal Source Of Atrial Fibrillation Treated By Discrete Radiofrequency Ablation” Circulation 95:572–576 (1997). At the site of arrhythmogenic tissue, in both right and left atria, several pulses of a discrete source of radiofrequency energy were applied in order to eliminate the fibrillatory process.
The treatment of reentrant wavelet arrhythmias through the use of catheter-based ablation techniques required the development of methods and devices for generating continuous linear lesions, like those employed in the maze procedure. Initially, conventional ablation tip electrodes were adapted for use in “drag burn” procedures to form linear lesions. During the “drag” procedure, as energy was being applied, the catheter tip was drawn across the tissue along a predetermined pathway within the heart. Alternatively, sequentially positioning the distal tip electrode, applying a pulse of energy, and then re-positioning the electrode along a predetermined linear pathway also made lines of ablation.
Subsequently, conventional catheters were modified to include multiple electrode arrangements. Such catheters typically contained a plurality of ring electrodes circling the catheter at various distances extending proximally from the distal tip of the catheter. More detailed examples of such catheter-based tissue ablation assemblies have been disclosed in U.S. Pat. No. 5,676,662 to Fleischhacker et al.; U.S. Pat. No. 5,688,267 to Panescu et al.; and U.S. Pat. No. 5,693,078 to Desai et al.
Further more detailed examples of transcatheter-based tissue ablation assemblies and methods are described in the following references: U.S. Pat. No. 5,575,810 to Swanson et al.; PCT Published Application WO 96/10961 to Fleischman et al.; U.S. Pat. No. 5,702,438 to Avitall; U.S. Pat. No. 5,687,723 to Avitall; U.S. Pat. No. 5,487,385 to Avitall; and PCT Published Application WO 97/37607 to Schaer.
While the disclosures above describe feasible catheter designs for imparting linear ablation tracks, as a practical matter, most of these catheter assemblies have been difficult to position and maintain placement and contact pressure long enough and in a sufficiently precise manner in the beating heart to successfully form segmented linear lesions along a chamber wall. Indeed, many of the aforementioned methods have generally failed to produce closed transmural lesions, thus leaving the opportunity for the reentrant circuits to reappear in the gaps remaining between point or drag ablations.
Due to the shortcomings associated with linear ablation techniques, a new method of treating atrial fibrillation was developed whereby a circumferential lesion is formed along a pulmonary vein ostium. The formation of a circumfential lesion creates a circumferential conduction block that electrically isolates a substantial portion of a posterior atrial wall from an arrhythmogenic focus located in a pulmonary vein. In a variation of this method, a circumferential lesion can be formed in combination with linear lesions to treat atrial fibrillation caused by wandering reentrant wavelets. These methods are disclosed in detail in U.S. Pat. No. 6,024,740 to Lesh.
U.S. Pat. No. 6,024,740 to Lesh et al. discloses a circumferential ablation device assembly used to form a circumferential lesion. The circumferential ablation device assembly includes an ablation element and an expandable member. The device is anchored in the pulmonary vein ostium using the expandable member and the ablation element is energized to form a circumferential lesion.
Although the aforementioned methods and devices have shown great success in treating atrial fibrillation through the formation of a circumferential lesion, optimizing the effectiveness of such methods and devices depends to some extent on the precise positioning of the ablation element at a location where the pulmonary vein extends from the atrium. At this time, minimal means have been disclosed for advancing ablation catheters to anatomic sites of interest such as the pulmonary veins.
Guidewire positioning techniques are known in the art and have been used extensively for catheter placement within difficult areas of a patient's vasculature. Guidewire positioning techniques generally involve advancing a guidewire through a patient's vasculature to the desired anatomical site and then advancing a catheter over the guidewire. However, the use of a guidewire alone does not provide an adequate means for placement of a catheter in a pulmonary vein because placement of the guidewire itself within a pulmonary vein poses a significant challenge.
Deflectable tip catheters are also known in the art and are often used for facilitating catheter placement. Deflectable tip catheters generally incorporate one or more internal pull wires affixed to the distal tip and to a proximal handle with a steering control mechanism. The steering control mechanism is used to deflect the tip of the catheter, usually in a single direction, as the catheter is advanced through a patient's vasculature. Detailed examples of steerable catheters and methods are described in the following references: U.S. Pat. No. 5,702,433 to Taylor, U.S. Pat. No. 5,755,327 to Randolph, U.S. Pat. No. 5,865,800 to Mirarchi et al., U.S. Pat. No. 5,882,333 to Schaer, U.S. Pat. No. 6,022,955 to Willems, U.S. Pat. No. 6,024,739 to Ponzi, U.S. Pat. No. 6,083,222 to Klein to Taylor,
Although deflectable tip catheters have been successful in addressing certain internal cardiac areas, existing deflectable tip catheter designs are not well-suited for advancing an ablation catheter into a pulmonary vein. In practice, it has been found that existing deflectable tip catheter designs are not capable of navigating the sharp angle from the fossa ovalis to the pulmonary vein without great difficulty. Furthermore, once the deflectable tip catheter reaches the pulmonary vein ostium, the ablation element is often unable to sustain sufficient contact with the surrounding tissue to create an adequate circumferential lesion.
Therefore, a need exists for an improved ablation catheter that can be advanced through a patient's vasculature to a pulmonary vein ostium in a quick and easy manner. It is also desirable that such an ablation catheter be capable of engaging the surrounding tissue to create a circumferential lesion for isolating a pulmonary vein from the posterior atrial wall of the heart. A device that achieves these objectives would represent a significant advancement in the treatment of atrial fibrillation.